The present invention relates to a method and apparatus for the transmission of PCM words by means of a plurality of memory arrangements each of which being associated with one PCM link and which are included in a PCM exchange for switching the PCM words from incoming channels in incoming links to outgoing channels in outgoing links via a bus arranged as a common time multiplex connection wherein the PCM words are transferred on the links in link time slots and on the bus in bus time slots; and the memory arrangements store, on the one hand, the PCM words before and after switching, and on the other hand switching information concerning the channels which cooperate in the incoming and outgoing links, respectively.
A time-multiplex-PCM-system is obtained if on each of n links q analog information signals are transferred, each information signal being allotted a link time slot with length or duration T1 within a frame period F. In standardized system F=125 .mu.s and q=32 information channels, i.e., T1.apprxeq.4 .mu.s. During each time slot the amplitude of the respective analog signal is indicated in coded form by a PCM word, for example by 8 pulses in series wherein each pulse has a negative or positive polarity to indicate one of two binary states. It follows that the pulses in the series are transferred in 0.5 .mu.s intervals, i.e., with a pulse repetition rate of 2 MHz. In a synchronous system full coincidence exists for the frames and time slots and pulses, respectively, of all links.
The function of a digital time multiplex exchange is to switch an incoming signal in a channel za during the link time slots tla on a link La so that the signal is outgoing on a link Lb in a channel zb during the link time slots tlb. Consequently it is typical that the space switching (change from link La to link Lb) is accompanied by a time switching (from time slot tla to time slot tlb).
It is known, for example, by the Swedish Pat. No. 351,541 (U.S. Pat. No. 3,864,525) how to carry out time switching by means of memory arrangements. In this connection word memories are used in order to store the PCM words during the time between the time slots tla and tlb. Each of the word memories which is associated with a link, has to be sufficiently large to store all PCM words within a frame, and comprises consequently for standardized systems 32 groups of 8 memory elements. Since in one and the same memory the write operations must never (inclusive the switching case tla=tlb) overlap the read operations, then, according to the above standardized system, there must be access time of 0.25 .mu.s, corresponding to a pulse base frequency fa=4 MHz so that the operations being controlled i.e., the writings and readings occur during the first and second halves of the time slots, respectively. The addressing operations of the word memories for reading and writing are controlled by means of a cyclical scanning arrangement and by means of a decoder which is fed with channel indices za, zb cyclically read from an index memory, as it will be described below.
While the time switching in this manner will be achieved completely without expensive gate multiplex switching arrangements the space stage of a PCM exchange comprises gate matrixes which are activated by means of control signals obtained from a control unit and the synchronization of the exchange often causing great problems. In said Swedish Pat. No. 351,541 an exchange is described which comprises one space stage arranged between two time stages (time-space-time principle). Because of the two time switchings executed in the two time stages the space switching can be made completely independent of the inward and outward time slots tla and tlb, respectively as will hereinafter become apparent.
Furthermore, there is described, for example, in "Colloque International de Commutation Electronique, Paris 1966" by an article "Switching, synchronising and signalling in PCM exchanges" of W. Neu and A. Kundig, how such space switching is avoided if the time switching is combined in such a way with a multiplex formation change, that all the r=n.multidot.q channels of the system are associated with just one common time multiplex connection, in the following denoted as a bus, so that there is allotted to each unit of information a bus time slot with length T2=t1/n. Even if it is assumed that the signals are transferred by means of parallel pulses on each bus which according to the standard example consists of 8 parallel lines, then according to the above analysis a pulse base frequency fb=1000.multidot.4/8=500 MHZ is obtained for, for example, n=1000 links. Since no presently known memories operate with 2 ns-access times, such systems with only time switching have heretofore been limited to comparatively small PCM exchanges.